Two-Layer Si-Carbon Negative Electrode for Cycle Stability
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Solution Overview
Problem
The degradation of charge-discharge cycle characteristics in nonaqueous electrolyte secondary batteries using Si-containing compounds as negative electrode active materials is exacerbated by volume changes, leading to increased isolation of active material particles from conductive paths, which degrades battery performance.
Innovation Solution
A negative electrode with a two-layer structure, where the first layer contains a Si-containing compound and carbon material A with polyacrylic acid as a binding agent, and the second layer contains a carbon material B with higher tap density and BET specific surface area, improving ion acceptance and wetting characteristics, thereby reducing particle isolation and enhancing battery performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the content of binding agent is increased to inhibit particle isolation, then charge-discharge cycle characteristics are improved, but resistance of negative electrode mixture layer increases and input characteristics are degraded
Solution Approach 1:
The negative electrode mixture layer is divided into two distinct layers: a first layer containing Si-containing compound with binding agent to maintain particle contact, and a second layer with high tap density carbon material to provide conductive paths and improve ion acceptance. This segmentation allows each layer to specialize in different functions, resolving the contradiction between particle stability and conductivity.
Solution Approach 2:
Different regions of the negative electrode mixture layer are assigned different compositions and properties. The first layer near the current collector has higher binding agent content for structural stability, while the second layer has higher carbon content and tap density for conductivity and ion acceptance, creating local optimization that resolves the global contradiction.
2Quantity of substance
If Si-containing compound is used as negative electrode active material to increase lithium ion storage capacity, then battery capacity is improved, but volume change during charging and discharging causes particle isolation and degrades charge-discharge cycle characteristics
Solution Approach 1:
The first layer is designed with binding agent (polyacrylic acid or its salt) to preemptively counteract the volume expansion and contraction of Si-containing compounds during lithium insertion and extraction. This binding agent maintains particle contact and prevents isolation before it occurs, cushioning against the harmful effects of volume change.
Solution Approach 2:
The negative electrode uses a composite structure combining Si-containing compound (for high capacity) with carbon materials (for structural stability and conductivity) and binding agents (for particle adhesion). This composite approach allows the Si-containing compound to provide high lithium ion storage capacity while the other components mitigate its volume change issues.
3Power
If carbon material with higher tap density is used in the second layer to improve ion acceptance, then input characteristics are improved, but the complexity of the negative electrode structure increases
Solution Approach 1:
Instead of trying to optimize a single-layer structure, the invention transitions to a two-dimensional layered structure. The second layer with higher tap density carbon material is added as a distinct layer above the first layer, utilizing the vertical dimension to introduce improved ion acceptance without fundamentally redesigning the entire electrode architecture.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration inhibits the degradation of charge-discharge cycle characteristics and improves input characteristics by maintaining high ion conductivity and capacity retention, while mitigating volume changes in the Si-containing compound.
Implementation Method 1
The carbon material B has a tap density higher than a tap density of the carbon material A. The carbon material B has a BET specific surface area higher than a BET specific surface area of the carbon material A
Data Source
Figure 1~2
AI summary
A negative electrode for a nonaqueous electrolyte secondary battery includes a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector. The negative electrode mixture layer includes a first layer and a second layer. The first layer is formed on the negative electrode current collector and includes a negative electrode active material and a first binding agent. The negative electrode active material in the first layer includes a carbon material A and a Si-containing compound. The first binding agent includes polyacrylic acid or a salt thereof. The second layer is formed on the first layer and includes a negative electrode active material and a second binding agent. The negative electrode active material in the second layer includes a carbon material B. The carbon material B has a tap density higher than a tap density of the carbon material A. A mass of the first layer relative to a mass of the negative electrode mixture layer is 50 mass% or greater and less than 90 mass%. A mass of the second layer relative to the mass of the negative electrode mixture layer is greater than 10 mass% and 50 mass% or less. A packing density of the second layer is lower than a packing density of the first layer.